International A Level Biology / Mitosis, meiosis and reproduction
Calculating the mitotic index
Edexcel International A Level Biology (YBI11), chapter 8: Mitosis, meiosis and reproduction, specification point 3.16. Free lesson.
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The mitotic index is the proportion of counted cells that are in mitosis. It helps you compare how quickly different tissues are growing, and you can use it to estimate cell numbers and time spent in a stage of the cell cycle.
What the mitotic index measures
The mitotic index is the number of cells in mitosis divided by the total number of cells counted. Count cells in prophase, metaphase, anaphase and telophase as being in mitosis. The total includes every cell counted, whether it is dividing or not. The index is a proportion, so it cannot be more than 1. Give it as a decimal unless the question asks for a percentage. Interphase cells are not included in the numerator, but they are included in the total.
How to recognise cells in mitosis
In a stained onion root tip, the chromosomes show up clearly. You need to identify each stage so that you count all dividing cells, not just those with chromosomes that are easy to see. Interphase: a round nucleus with no separate chromosomes. Most cells are in this stage. Prophase: chromosomes appear as threads inside the nucleus. Metaphase: chromosomes form a band across the middle of the cell. Anaphase: two groups of chromatids move towards opposite ends of the cell. Telophase: two new cells form, with a thin new wall between them. An edge cell is only partly visible,…
How to calculate the index and work back to cell numbers
To calculate the index, add the cells in all stages of mitosis, then divide by the total counted. If you are given the index and total instead, multiply them to find the number of cells in mitosis. To find interphase cells, subtract that number from the total.
What the index tells you about growth
A high index means a large proportion of cells are dividing, so the tissue is growing quickly. In an onion root, the index is about 0.15 around 1 mm from the tip, in the meristem where cells divide rapidly. By 4 mm it is about 0.01, where cells are getting longer rather than dividing. Tumours can also have a high index because their cells divide out of control.
How to make the count reliable
A reliable index depends on a fair count. A busy-looking field can give a result that is too high, so count several random fields in the meristem. Use consistent rules and count enough cells to reduce the effect of chance. Edge cells: count cells crossing two chosen edges, such as the top and left, and ignore cells crossing the other two. Field choice: choose several random fields within the meristem, rather than selecting a busy-looking patch. Unclear cells: early prophase can resemble interphase, so set a rule first and apply it to every cell. Sample size: count many cells…